The present invention relates to a fire alarm system including a plurality of fire detectors connected to power supply and signal lines connected to a central receiving unit (reciever) whereby when a fire breaks out, the location of the fire is discriminated by the receiver in accordance with the specified address of any detector that has given an alarm.
In the case of a large-scale fire detecting system covering a wide area under surveillance, its equipment construction is such that a plurality of repeaters are connected to a main circuit extended from a receiver and a plurality of fire detectors are connected to a branch circuit extended from each of the repeaters.
On the other hand, in the case of a fire alarm system of a medium scale or less covering an area under surveillance which is not so wide, its equipment construction is such that the previously mentioned repeaters are not provided and a plurality of detectors are connected to each of a plurality of circuits extended from a receiver.
Then, recently a system has been developed in which in order to discriminate and indicate an area on fire by a receiver, a specified address signal is assigned to each of a plurality of addressable detectors such that such specified address is sent, along with a fire detection signal, to the receiver from any alarming detector.
However, with the fire alarm system in which all of the detectors are each assigned with its own specified address signal, it is necessary that the plurality of detectors installed within the same area under surveillance such as the same floor of a building must respectively be assigned with different address signals and that for the system on the whole every one of the detectors must be provided with a circuit function to send its own address signal to the receiver upon detection of a fire, thus increasing the equipment cost of the fire alarm system and thereby greatly preventing the practical use of a function of discriminating the indicating areas on fire by the receiver.
Also, where a specified address signal is assigned to every one of the detectors, there is the disadvantage that the area under surveillance will be limited excessively if there is a limitation to the number of addresses, whereas the burden on the signal discriminating capacity of the receiver will be increased if the number of addresses is excessively large.
Moreover, when the number of the alarming detectors is increased by the spreading of the fire after its outbreak, as the number of the alarming detectors connected to the same circuit is increased, the current flowing in the circuit is increased and the supply voltage to the detectors is decreased due to the effect of the voltage drop caused by the line resistance. As a result, the clock pulse voltage for sending the fire detection information and the specified address information from each alarming detector to the receiver becomes in insufficient and eventually a point is reached where there is the danger of the transmission of information being disabled. This deteriorates the advantage of the fire alarm system employing the addressable detectors that the progress of the spread of the fire can also be grasped moment by moment on the receiver side.